1. How to represent a document?

2. What are vector space and bag-of-words models?

3. Features in text? And how to do text feature selection?

4. How to classify text data?

5. How to evaluate a classifier?

• Supervised learning: Learning a function that maps an input to an output based on example input-output pairs.

  • infer a function from labeled training data

  • use the inferred function to label new instances

• Human experts annotate a set of text data

  • Training set

• Assigning subject categories, topics, or genres

• Spam detection

• Authorship identification

• Age/gender identification

• Language Identification

• Sentiment analysis

• …

• Which problem is not a text classification task? (less likely to be)

  • Author’s gender detection from text

  • Finding about the smoking conditions of patients from clinical letters

  • Grouping news articles into political vs non-political news

  • Classifying reviews into positive and negative sentiment

Go to www.menti.com and use the code 86 08 86 5

• Represent by a string?

  • No semantic meaning

• Represent by a list of sentences?

  • Sentence is just like a short document (recursive definition)

• Represent by a vector?

  • A vector is an ordered finite list of numbers.

• A vector space is a collection of vectors

• Represent documents by concept vectors

  • Each concept defines one dimension

  • k concepts define a high-dimensional space

  • Element of vector corresponds to concept weight

• Distance between the vectors in this concept space

  • Relationship among documents

• The process of converting text into numbers is called Vectorization

• Terms are generic features that can be extracted from text

• Typically, terms are single words, keywords, n-grams, or phrases

• Documents are represented as vectors of terms

• Each dimension (concept) corresponds to a separate term

\[d = (w_1, ..., w_n)\]

• The Corpus represents a collection of documents (the dataset)

• The Vocabulary is the set of all unique terms in the corpus

• All documents are projected into this concept space

• Bag of Words

• Topics

• Word Embeddings

• With Bag of Words (BOW), we refer to a Vector Space Model where:

  • Terms: words (more generally we may use n-grams, etc.)

  • Weights: number of occurrences of the terms in the document

• Term as the basis for vector space

  • Doc1: Text mining is to identify useful information.

  • Doc2: Useful information is mined from text.

  • Doc3: Apple is delicious.

• Zipf’s law

  • Frequency of any word is inversely proportional to its rank in the frequency table
  • Formally
    • \(f(k;s,N)=\frac{1/k^S}{\sum^N_{n=1}{1/n^S}}\)
      where \(k\) is rank of the word; \(N\) is the vocabulary size; \(s\) is language-specific parameter
  • Simply: \(f(l;s,N)\propto1/k^S\)

Source:https://en.wikipedia.org/wiki/Zipf's_law

• In a clinical text dataset, if we know the most popular word’s frequency is 69,542, what is your best estimate of its second and third most popular word’s frequencies respectively?

Zipf’s law tells us:

• Head words take large portion of occurrences, but they are semantically meaningless E.g., the, a, an, we, do, to
• Tail words take major portion of vocabulary, but they rarely occur in documents E.g., Pneumonoultramicroscopicsilicovolcanoconiosis (45 letters, a lung disease)
• The rest is most representative

• Binary

  • with 1 indicating that a term occurred in the document, and 0 indicating that it did not

• Idea: a term is more important if it occurs more frequently in a document

• TF Formulas

  • Let \(t(c,d)\) be the frequency count of term \(t\) in doc \(d\)

  • Raw TF: \(tf(t,d) = c(t,d)\)

• Idea: a term is more discriminative if it occurs a lot but only in fewer documents

Let \(n_{d,t}\) denote the number of times the \(t\)-th term appears in the \(d\)-th document.

\[TF_{d,t} = \frac{n_{d,t}}{\sum_i{n_{d,i}}}\] Let \(N\) denote the number of documents annd \(N_t\) denote the number of documents containing the \(t\)-th term.

\[IDF_t = log(\frac{N}{N_t})\] TF-IDF weight:

\[w_{d,t} = TF_{d,t} \cdot IDF_t\]

• Euclidean distance

  • \(dist(d_i, d_j) = \sqrt{\sum_{t\in V}{[tf(t,d_i)idf(t) - tf(t, d_j)idf(t)]^2}}\)

  • Longer documents will be penalized by the extra words

  • We care more about how these two vectors are overlapped

• Feature selection is the process of selecting a specific subset of the terms of the training set and using only them in the classification algorithm.

• high dimensionality of text features

• Select the most informative features for model training

  • Reduce noise in feature representation

  • Improve final classification performance

  • Improve training/testing efficiency

    • Less time complexity

    • Fewer training data

• Wrapper method

  • Find the best subset of features for a particular classification method
  • Sequential forward selection or genetic search to speed up the search

• Filter method

  • Evaluate the features independently from the classifier and other features

    • No indication of a classifier’s performance on the selected features

    • No dependency among the features

  • Feasible for very large feature set

    • Usually used as a preprocessing step

• Rare words: non-influential for global prediction, reduce vocabulary size

• Decrease in entropy of categorical prediction when the feature is present or absent

Let \(p(c | t)\) be the conditional probability that a document belongs to class \(c\), given the fact that it contains the term \(t\). Therefore, we have:

\[\sum^k_{c=1}{p(c | t)=1}\]

Then, the gini-index for the term \(t\), denoted by \(G(t)\) is defined as:

\[G(t) = \sum^k_{c=1}{p(c | t)^2}\]

• The value of the gini-index lies in the range \((1/k, 1)\).

• Higher values of the gini-index indicate a greater discriminative power of the term t.

• If the global class distribution is skewed, the gini-index may not accurately reflect the discriminative power of the underlying attributes.

• Normalized gini-index
• Mutual Information

• \({\chi}^2\)-Statistic

• Input:

  • A training set of \(m\) manually-labeled documents \((d_1,c_1),\cdots,(d_m,c_m)\)

  • a fixed set of classes \(C = \{c_1, c_2,…, c_J\}\)

• Output:

  • a learned classifier \(y:d \rightarrow c\)

• Rules based on combinations of words or other features

• Accuracy can be high: If rules carefully refined by expert

• But building and maintaining these rules is expensive

• Data/Domain specifics

• Logistic regression
• K-nearest neighbors
• Naïve Bayes
• Support vector machines
• Neural Networks

Each class is represented by its centroid, with test samples classified to the class with the nearest centroid. Using a training set of documents, the Rocchio algorithm builds a prototype vector, centroid, for each class. This prototype is an average vector over the training documents’ vectors that belong to a certain class.

\[\boldsymbol{\mu_c} = \frac{1}{|D_c|}\sum_{\mathbf{d} \in D_c}{\mathbf{d}}\]

Where \(D_c\) is the set of documents in the corpus that belongs to class \(c\) and \(d\) is the vector representation of document \(d\).

The predicted label of document d is the one with the smallest (Euclidean) distance between the document and the centroid.

\[\hat{c} = \arg \min_c ||\boldsymbol{\mu_c} - \mathbf{d}||\]

• Given a test document d,. the KNN algorithm finds the k nearest neighbors of d among all the documents in the training set, and scores the category candidates based on the class of the k neighbors. After sorting the score values, the algorithm assigns the candidate to the class with the highest score.

• The basic nearest neighbors classification uses uniform weights: that is, the value assigned to a query point is computed from a simple majority vote of the nearest neighbors. Under some circumstances, it is better to weight the neighbors such that nearer neighbors contribute more to the fit.

• Simple (“naïve”) classification method based on Bayes rule

• Relies on very simple representation of document

  • Bag of words

• For a document \(d\) and a class \(c\)

\[P(c|d) = \frac{P(d|c)P(c)}{P(d)}\]

\[P(x_1, x_2, \ldots, x_n|c)\]

• Bag of Words assumption: Assume position doesn’t matter

• Conditional Independence: Assume the feature probabilities \(P(x_i|c_j)\) are independent given the class \(c\).

\[P(x_1, \ldots, x_n|c) = P(x_1 | c) \cdot P(x_2|c) \cdot P(x_3|c) \cdot \ldots \cdot P(x_n|c)\]

\[C_{MAP} = \underset{c \in C}{\operatorname{argmax}}P(x_1, x_2, \ldots,x_n|c)P(c)\]

\[C_{NB} = \underset{c \in C}{\operatorname{argmax}}P(c_j)\prod_{x \in X}{P(x|c)}\] \[C_{NB} = \underset{c \in C}{\operatorname{argmax}}P(c_j)\prod_{i \in positions}{P(x_i|c_i)}\]

• First attempt: maximum likelihood estimates

  • simply use the frequencies in the data

\[\hat{P}(c_j) = \frac{doccount(C = c_j)}{N_{doc}}\] \[\hat{P}(w_i|c_j) = \frac{count(w_i, c_j)}{\sum_{w \in V}count(w, c_j)}\]

fraction of times word \(w_i\) appears among all words in documents of topic \(c_j\)

• Create mega-document for topic \(j\) by concatenating all docs in this topic

  • Use frequency of \(w\) in mega-document

What if we have seen no training documents with the word fantastic and classified in the topic positive (thumbs-up)?

\[\hat{P}(\mathrm{"fantastic"|positive}) = \frac{count(\mathrm{"fantastic", positive})}{\sum_{w \in V}{count(\mathrm{w,positive})}}\] Zero probabilities cannot be conditioned away, no matter the other evidence!

\[C_{MAP} = \underset{c}{\operatorname{argmax}}\hat{P}(c)\prod_i{\hat{P}(x_i|c)}\]

\[ \begin{align} \hat{P}(w_i|c) &= \frac{count(w_i,c)+1}{\sum_{w \in V}{(count(w,c)+1)}} \\ &= \frac{count(w_i,c)+1}{(\sum_{w \in V}{count(w,c) + |V|})} \end{align} \]

• From training corpus, extract Vocabulary

Add one extra word to the vocabulary, the “unknown word” \(w_u\)

\[ \begin{align} \hat{P}(w_u|c) &= \frac{count(w_u,c)+1}{(\sum_{w \in V}{count(w,c)})+|V+1|} \\ &= \frac{1}{(\sum_{w \in V}{count(w,c)}) + |V+1|} \end{align} \]

• The main principle of SVM is to determine separators in the search space which can best separate the different classes.
• SVM tries to make a decision boundary in such a way that the separation between the two classes is as wide as possible.

• It is not necessary to use a linear function for the SVM classifier.

• With the kernel trick, SVM can construct a nonlinear decision surface in the original feature space by mapping the data instances non-linearly to a new space where the classes can be separated linearly with a hyperplane.

• In practice, linear SVM is used most often because of their simplicity and ease of interpretability.

• SVM is quite robust to high dimensionality.

• A decision tree is a hierarchical decomposition of the (training) data space, in which a condition on the feature value is used in order to divide the data space hierarchically.

• Top-down, by choosing a variable at each step that best splits the set of items.
• Different algorithms to measure the homogeneity of the target variable within the subsets.
  • Gini impurity
  • Information gain

• Random forests are an ensemble learning method for classification, regression and other tasks that operates by constructing a multitude of decision trees at training time.
• Fit multiple trees to bootstrapped samples of the data AND at each node select best predictor from only a random subset of predictors. Combine all trees to yield a consensus prediction

• Training set

• Test set

• Validation set (dev set)

  • A validation dataset is a dataset of examples used to tune the hyperparameters (i.e. the architecture) of a classifier. It is sometimes also called the development set or the “dev set”.

https://scikit-learn.org/stable/modules/cross_validation.html

• What proportion of instances is correctly classified?
  TP + TN / TP + FP + FN + TN

• Accuracy is a valid choice of evaluation for classification problems which are well balanced and not skewed.

• Let us say that our target class is very sparse. Do we want accuracy as a metric of our model performance? What if we are predicting if an asteroid will hit the earth? Just say “No” all the time. And you will be 99% accurate. The model can be reasonably accurate, but not at all valuable.

• Precision: % of selected items that are correct
  Recall: % of correct items that are selected

• Precision is a valid choice of evaluation metric when we want to be very sure of our prediction.

• Recall is a valid choice of evaluation metric when we want to capture as many positives as possible.

A combined measure that assesses the P/R tradeoff is F measure (weighted harmonic mean):

\[F = \frac{1}{\alpha\frac{1}{P}+(1-\alpha)\frac{1}{R}}=\frac{(\beta^2+1)PR}{\beta^2P + R}\]

The harmonic mean is a very conservative average; see IIR § 8.3

People usually use balanced F1 measure - i.e., with \(\beta = 1\) (that is, \(\alpha = 1/2\)): \(F = 2PR/(P+R)\)

• AUC provides an aggregate measure of performance across all possible classification thresholds.

• One way of interpreting AUC is as the probability that a classifier will rank a randomly chosen positive instance higher than a randomly chosen negative one (assuming ‘positive’ ranks higher than ‘negative’). AUC measures how well predictions are ranked, rather than their absolute values.

• AUC ranges in value from 0 (100% wrong), to 0.5 (random classifier), to 1 (100% correct). So, for example, if you as a marketer want to find a list of users who will respond to a marketing campaign, AUC is a good metric to use since the predictions ranked by probability is the order in which you will create a list of users to send the marketing campaign.

• Gee, I’m building a text classifier for real, now!

• What should I do?

If (wheat or grain) and not (whole or bread) then Categorize as grain

• Need careful crafting

  • Human tuning on development data

  • Time-consuming: 2 days per class

• Use Naïve Bayes

  • Naïve Bayes is a “high-bias” algorithm (Ng and Jordan 2002 NIPS)

• Get more labeled data

  • Find clever ways to get humans to label data for you

• Try semi-supervised training methods:

  • Bootstrapping, EM over unlabeled documents, …

• Perfect for all the clever classifiers

  • SVM

  • Regularized Logistic Regression

• You can even use user-interpretable decision trees

  • Users like to hack

  • Management likes quick fixes

• Can achieve high accuracy!

• At a cost:

  • SVMs (train time) or kNN (test time) can be too slow

  • Regularized logistic regression can be somewhat better

• So Naïve Bayes can come back into its own again!

• Domain-specific features and weights: very important in real performance

• Sometimes need to collapse terms:

  • Part numbers, chemical formulas, …

  • But stemming generally doesn’t help

• Upweighting: Counting a word as if it occurred twice:

  • title words

  • first sentence of each paragraph (Murata, 1999)

  • In sentences that contain title words

• Corpus: is a large and structured set of texts

• Stop words: words which are filtered out before or after processing of natural language data (text)

• Unstructured text: information that either does not have a pre-defined data model or is not organized in a pre-defined manner.

• Tokenizing: process of breaking a stream of text up into words, phrases, symbols, or other meaningful elements called tokens (see also lexical analysis)

• Natural language processing: field of computer science, artificial intelligence, and linguistics concerned with the interactions between computers and human (natural) languages

• Term document (or document term) matrix: is a mathematical matrix that describes the frequency of terms that occur in a collection of documents

• Supervised learning: is the machine learning task of inferring a function from labeled training data

• Unsupervised learning: find hidden structure in unlabeled data

• Vector space model & BOW

• Feature Selection

• Text Classification

• Evaluation